Create video and audio decoder threads on demand.

media/filters/ffmpeg_video_decoder.h

Index: media/filters/ffmpeg_video_decoder.h
diff --git a/media/filters/ffmpeg_video_decoder.h b/media/filters/ffmpeg_video_decoder.h
index 1e9f74d9fa3fc3384e2835d0594af86dfcdacaae..ede7e11b643406dbb013afd13823472d0e2af5f6 100644
--- a/media/filters/ffmpeg_video_decoder.h
+++ b/media/filters/ffmpeg_video_decoder.h
@@ -19,9 +19,12 @@ struct AVFrame;
 
 namespace media {
 
+class MessageLoopFactory;
+
 class MEDIA_EXPORT FFmpegVideoDecoder : public VideoDecoder {
  public:
-  explicit FFmpegVideoDecoder(MessageLoop* message_loop);
+  FFmpegVideoDecoder(MessageLoopFactory* loop_factory,
+                     const std::string& loop_name);
   virtual ~FFmpegVideoDecoder();
 
   // Filter implementation.
@@ -68,6 +71,11 @@ class MEDIA_EXPORT FFmpegVideoDecoder : public VideoDecoder {
   // the current state of |codec_context_|.
   scoped_refptr<VideoFrame> AllocateVideoFrame();
 
+  MessageLoop* EnsureMessageLoop();
+
+  MessageLoopFactory* message_loop_factory_;
+  const std::string loop_name_;
+
   MessageLoop* message_loop_;
 
   DecoderState state_;